1. Field of the Invention
The present invention relates to radiation/X-ray imaging systems, such as medical X-ray/radiation imaging systems, X-ray inspection systems, X-ray analysis systems, and radiation monitoring systems, and to semiconductor radiation detectors used in these systems.
2. Background Art
Cases in which an X-ray detector, which is one example of a semiconductor radiation detector, is applied to a multi-slice X-ray CT scanner will be hereafter described as a representative example. An X-ray CT scanner is an apparatus with which imaging is conducted on an object in a plurality of directions, and it calculates X-ray attenuation coefficients based on the projection, so as to obtain cross-sectional views of the object. Such X-ray CT scanner is widely used in the field of medicine or nondestructive testing. Patent Document 1 is a typical example thereof. In order to realize such imaging, for example, an X-ray tube and an X-ray detector are disposed so that they sandwich the object, and they are mounted on a rotated gantry so that they can move around the object. This X-ray detector is composed of a plurality of X-ray detecting elements that are disposed approximately in an arc having the X-ray tube as its center. A system (indirect type) in which an X ray is detected by a scintillator so as to covert it into light, and the light is then converted into an electric signal by a silicon wafer or the like is mainly used for the X-ray detecting elements.
Further, regarding the multi-slice X-ray CT scanner, a plurality of stages of X-ray detecting element rows are also formed in the direction of the axis of rotation (in the direction of slice). Use of such X-ray detector in which the X-ray detecting elements are disposed in a two dimensional manner enables imaging with a wide field of view in the direction of the axis of rotation in one imaging or enables imaging with a wide field of view in a short amount of time, and therefore, the multi-slice X-ray CT scanners are increasingly widespread.
As shown in Patent Document 2, regarding the X-ray detectors used for the multi-slice X-ray CT scanners, a system in which the X-ray detecting elements are provided with readout circuits on a one-on-one basis has been conventionally used. However, because the number of circuits is increased as the multi-slice type is increasingly widespread as described above, as a structure that can prevent such increase, X-ray CT scanners equipped with X-ray detectors of a sequential readout system as indicated in Patent Document 3 have also been proposed. In this system, a capacitor for accumulating electric signals for each of the X-ray detecting elements and a switch for outputting the electron signals are provided, and the X-ray detecting elements in the same column (channel) are connected to the same readout circuit. With this structure, by turning on the switch, signals from the X-ray detecting elements in one row (slice) are obtained, and by sequentially changing the row, the elements to be read are switched, whereby signals from the X-ray detecting elements in different rows can be obtained from a common readout circuit. Thus, such structure in which a readout circuit is commonly used provides the advantageous effect of reading signals with a fewer readout circuits, as compared with conventional systems.
Meanwhile, based on multi-slice, since a sufficiently wide field of view can be realized even with pixel elements having a small size in the slice direction, the size of pixel elements in the slice direction is increasingly reduced. For example, while the size of a pixel element in the slice direction was on the order of 10 mm in the case of a single slice, it is on the order of 1 mm in the case of 16 slices or the like. For example, as described in Patent Document 4, in a reconstructed image, spatial resolution equal both in the slice direction and the channel direction is about to be realized. However, since signals generated from one pixel element are reduced along with such decrease in the size of pixel elements, the SNR is decreased and image quality is decreased due to the influence of circuit noise particularly at the time of low dose. As a method for preventing this, a direct-type detector that directly converts X rays into electric signals is applied. Based on this detector, by using amorphous selenium (a-Se), cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe), mercuric iodide (HgI2), or the like as a photoelectric conversion material that converts X rays into electric signals, it becomes possible to generate more electric signals per X-ray photon than an indirect-type detector. For example, Patent Document 5 is a typical example of this direct-type X-ray CT scanner.
In these X-ray detectors, generally, many X-ray detecting elements are formed on a semiconductor module or a module comprising a semiconductor layer. Based on such structure, since the X-ray detecting elements are adjacent to one another, separation between the elements is insufficient. Thus, due to voltage nonuniformity in the semiconductor module or the semiconductor layer, electrical charge drift, or the like, there are cases in which the inflow of signals from adjacent pixel elements or peripheral pixel elements is caused. This is referred to as “cross-talk.” Particularly, in the case of the direct type, since a relatively thick semiconductor module or semiconductor layer is necessary for detecting X rays, such cross-talk is easily caused. As a method for preventing this, Patent Document 2 proposes a structure in which a groove is provided between X-ray detecting elements so as to realize the separation (guard-ring) of X-ray detecting elements. Patent Document 6 proposes a structure for substantially realizing guard-ring in order to obtain a similar effect; that is, the periphery of the electrodes of X-ray detecting elements is provided with a guard-ring electrode, instead of a groove, and a voltage is made uniform thereby in the semiconductor module or the semiconductor layer. In this way, even when electrical charge drift is caused, before flowing into other pixel elements, it is read by the guard-ring electrode guard-ring. Further, Patent Document 7 proposes a structure to remove or reduce the distortion of the electric fields in the X-ray detecting elements located at the end portions and prevent the inflow of signals from ineffective regions by providing a guard-ring electrode so that it surrounds the entire X-ray detecting region. Based on these guard-ring structures, the cross-talk or leak currents from ineffective regions can be reduced.
Patent Document 1: JP Patent Publication (Kokai) No. 2001-242253 A
Patent Document 2: JP Patent Publication (Kokai) No. 2000-316841 A
Patent Document 3: JP Patent Publication (Kokai) No. 2003-7275 A
Patent Document 4: JP Patent Publication (Kokai) No. 2004-24659 A
Patent Document 5: JP Patent Publication (Kokai) No. 7-333348 A (1995)
Patent Document 6: JP Patent Publication (Kokai) No. 2005-159156 A
Patent Document 7: JP Patent Publication (Kokai) No. 2005-57281 A